1. Field of the Invention
The present invention relates to an image recording apparatus of the type in which a latent image is formed on a latent image carrier, developed into a toner image, and the toner image is transferred onto a recording medium or an intermediate image transfer body, and an image recording method used in the apparatus. More particularly, the invention relates to an image recording apparatus based on electrophotographic image recording technique, electrostatic recording technique, ionography, magnetography or the like, and an image recording method used in the image recording apparatus.
2. Description of the Related Art
The indirect image transfer type image recording technique records an image on a recording paper in a manner that a toner image is formed on an image carrier, and the formed image is transferred onto the paper and then fixed onto the same. Generally, residual toner is collected which is still left on the image carrier after the toner image is transferred onto the paper, and a user disposes of the collected toner.
To record an image, the indirect transfer type image recording technique performs a process consisting of the following steps oft
1) charging uniformly the surface of an image carrier, which is coated with a photosensitive material;
2) exposing the uniformly charged surface of the image carrier to light containing image information to form an electrostatic latent image thereon;
3) developing the electrostatic latent image into a toner image;
4) transferring the toner image onto a recording paper;
0 5) fixing the toner image onto the paper by fusing toner; and
6) cleaning the surface of the image carrier by removing the toner that is still left on the surface of the image carrier after the transferring step ends.
In the cleaning step, the residual toner is removed with an elastic rubber blade or a brush in such a way that it is pressed against the surface of the image carrier, and the removed toner is collected in a container and periodically cast.
In this type of the image forming apparatus, an amount of toner accumulatively stored in the container is constantly detected and measured. Before the container is filled up with the collected toner, the toner has to be cast or the old container has to be replaced with a new empty container. In case where the image recording apparatus is reduced in size, it is difficult to secure a large space for receiving the collected toner container. Because of this, the inner space of a drum-like image carrier Is frequently used for storing the collected toner. This necessitates the setting up of a period of replacing the image carrier with a new one on the basis of the amount of the collected toner.
An attempt to reuse the collected toner has been made in view of the environment protection. However, the attempt involves many problems of sorting and transporting of the collected toner, energy for toner reuse, gathering ways and sites of cast toner, and the like.
The followings are possible solutions to the problems.
1) A first solution is to improve the efficiency of transferring the toner image onto the recording medium. If the transferring efficiency is improved, the amount of the residual toner on the image carrier is correspondingly reduced and the amount of toner to be collected and cast is also reduced.
2) A second solution is to return the collected residual toner to the developer, and to use it for the development again. Since the collected toner is completely reused, there is eliminated the work to cast the toner.
3) A third solution is such that the cleaning of the image carrier is not performed, and unwanted phenomena, for example, ghost, caused by the residual toner is removed by another means. If the unwanted phenomena by the residual toner are removed, it is not necessary to clean the image carrier for the residual toner removal, and the toner to be cast is not produced.
Conventional techniques for improving the transferring efficiency are listed below.
a) Unexamined Japanese Patent Publication (kokai) No. Sho. 56-126872 discloses that""s
an area where an electric field for transferring is formed is increased to improve the transferring efficiency.
b) Unexamined Japanese Patent Publication (kokai) Nos. Sho. 58-88770 and 58-140769 disclose that:
an alternative electric field is formed at the image transferring location, and this field applies a force so that the toner on the image carrier is swingably moved to accelerate the removal of the toner from the image carrier.
c) Unexamined Japanese Patent Publication (kokai) No. Sho. 52-126230 discloses that""s
an ultrasonic wave is projected onto the image transferring area on the image carrier, and with the ultrasonic wave, the image transferring area is vibrated to reduce the adhesion of toner particles.
d) Unexamined Japanese Patent Publication (kokai) Nos. Hei. 2-1870, 2-81053, 2-118671, 2-118672, and 2-157766 disclose that:
Separable fine particles of silica or the like are contained in the developing agent so that those fine particles intervene between the toner and the photoreceptor. Therefore, the adhesion acting between the toner and the photoreceptor is reduced to improve the transferring efficiency.
e) Unexamined Japanese Patent Publication (kokai) No. Hei. 1-134485:
For the latent image development, colorless and transparent toner is applied to a latent image formed on the image carrier, and colored toner is applied to the resultant latent image. In the thus formed toner image, the color toner particles are transferred almost perfectly.
The above techniques (a), (b) and (c) succeed in improving the transferring efficiency. However, some amount of toner is still left on the image carrier after the image transferring step ends. Accordingly, these techniques fail to provide a perfect solution to the problem of reducing the amount of casting toner.
In the technique (d), it is required to add a sufficient amount of separable fine particles to the developing agent and to coat toner particles with the separable fine particles. Actually, it is difficult to perfectly and uniformly coat all the toner particles with the separable fine particles. And, it is difficult to completely remove the tone particles imperfectly coated with the separable fine particles. Even if all the toner particles are uniformly coated with the separable fine particles, the separable fine particles often separate from the toner particles when those toner particles covered with the separable fine particles undergo various types of stresses, such as agitation, controlling of the layer thickness and the like. To maintain such a state that all the toner particles are uniformly coated with the separable fine particles, a developing unit must be constructed such that the toner particles coated with the separable fine particles are free from stress. Since a large number of separable fine particles are added to the toner, when the toner is used for a long time, the separable fine particles adhere to the surface of toner and carrier, the charging characteristic of the developing agent degrades, and the separated fine particles gather into agglomeration. This degrades the fluidity of the developing agent, possibly causing an irregular development. The toner containing a large amount of separable fine particles has a good fluidity. Accordingly, when a toner image contacts with an image transfer body in the image transferring process, the toner image tends to lose its shape. The deformation of the image caused by the image transferring process tends to occur.
In the technique (e), after the transferring of the toner image, much colorless and transparent toner is left on the surface of the image carrier. The surface of the image carrier before the next image forming process have to be cleaned so that a uniform surface of the image carrier is secured. The work to collect the residual toner by the cleaning unit and cast the collected one is essential. Accordingly, the technique (d) can not solve the problem of reducing the amount of the toner to be cast.
The followings are conventional techniques for reusing the collected toner referred to (2).
1) Unexamined Japanese Patent Publication (kokai) No. Sho. 54-121133 discloses that
toner collected by the cleaning unit is returned to the developing unit by way of a transporting path, and used again for developing latent images.
2) Unexamined Japanese Patent Publication (kokai) No. Sho. 53-125027 discloses that:
the cleaning unit and the developing unit are combined into a single unit, and toner collected by the cleaning unit is caused to fall into or transported to a container containing toner for development.
The technique for collecting residual toner on the image carrier by the developing unit, not using the cleaning unit is disclosed in Unexamined Japanese Patent Publication (kokai) Nos. Sho. 54-109642, 59-133573, 59-157661 and the like.
In the image recording apparatuses based on the above technique, after the transferring process of the toner image, the toner left on the background portion is collected by transferring the toner onto the developing roll In the electric field in the developing region before the next image is developed.
In the image recording apparatuses disclosed in Unexamined Japanese Patent Publication (kokai) Nos. Sho. 54-121133, 53-125027, 54-109842, 59-133573, 59-157661, the collected toner is not accumulatively stored. The paper powder, which is mixed with the toner during the image transferring process, is also collected by the developing unit. This causes an image defective. When the toner is repeatedly used, the charging characteristic of the toner varies, so that the image density is often instable. For this reason, the developing agent contained in the developing unit must be replaced with new one. In this case, the old toner must be cast. In the case of the image recording apparatus which additionally uses a device for transporting the toner to the developing unit, the apparatus structure is complicated.
The conventional technique not using the cleaning process is disclosed in Unexamined Japanese Patent Publication (kokai) Nos. Hei. 3-172880 and Sho. 64-20587.
If the image carrier is not cleaned after the image transferring process, a positive ghost and a negative ghost may appear on the recorded image. The positive ghost is formed when the residual toner is printed out in the next image forming process. The negative ghost is formed by the light interruption by the residual toner. In Unexamined Japanese Patent Publication (kokal) No. Hei. 3-172880, in order to prevent the ghost, the toner transferring efficiency is set at 80% or larger. In Unexamined Japanese Patent Publication (kokai) No. Hei. 3-114063, the amount of residual toner is set at 0.35 mg/cm2 or less, to thereby prevent the ghost.
In the technique disclosed in Unexamined Japanese Patent Publication (kokai) No. Sho. 64-20587, after the image transferring process, the residual toner is agitated with a brush or the like to thereby prevent the formation of the ghost by the residual toner.
To suppress the formation of the ghost and fog in those apparatuses, the transferring efficiency has to be improved.
It is seen that the transferring efficiency has to be improved in order to reduce the toner to be collected and cast to the smallest possible amount.
Further, in order to reduce the toner to be cast to zero, it is necessary to improve the transferring efficiency so that the formation of image defects, such as ghost and fog, are suppressed without the cleaning operation for removal of the residual toner that follows the transferring process.
It is an object of the present invention to reduce the amount of the toner to be collected and cast by improving the efficiency of transferring the toner image onto a recording sheet or an intermediate image transfer body so as to eliminate the use of the cleaning unit, and to simplify the construction of the image recording apparatus and to suppress the occurrence of the toner to be cast so as to maintain a high transferring efficiency stably for a long time.
An image recording apparatus according to the present invention comprises: an image carrying unit having a surface on which an electrostatic latent images is formed; a developing unit for developing the electrostatic latent image into a toner image by selectively applying toner to the surface of the image carrying unit; and an image transferring unit for transferring the toner image onto a recording sheet or an intermediate image transferring body; wherein fine particles are uniformly formed on the surface of the image carrying unit, the particle diameter of the fine particles being smaller than that of the toner, and the toner is transferred onto the fine particles on the surface of the image carrying unit.
An Image recording method comprises the steps of: forming an electrostatic latent image defined by electrostatic potential differences on image carrying unit having a uniform layer of fine particles formed on the surface thereof, the particle diameter of the fine particles being smaller than that of the toner; forming a toner image by selectively applying toner onto the layer of the fine particles; and transferring the toner image to a recording shoot or an intermediate image transferring unit.
In the indirect-transfer type printing apparatus or method of the present invention, a fine particle layer is uniformly formed on the image carrier before the toner image formation, and a toner image is formed on the fine particle layer. This unique feature brings about advantages. A first advantage is to considerably improve a toner image transfer rate. A second advantage is to reduce the toner collected from the image carrier having undergone the image transfer process to zero or considerably reduce the same.